1. Field of the Invention
The present invention relates to a slip-detecting device which is capable of detecting a slip of a driving belt of a generator.
2. Description of the Related Art
When a generator is driven by a driver, such as an internal combustion engine, via a driving belt, such as V-belt, it is important to correctly detect a slip of the driving belt.
In general, when the slip of the driving belt occurs, since a driving power of the driver is not adequately transferred to the generator, the output power of the generator decreases. In a generator used for a vehicle, this slip of the driving belt sometimes causes improper charging and results in damage to the battery. Therefore, it is necessary to detect the slip of the driving belt in order to prevent damage to the battery. Further, it is known that the remaining service life of the driving belt has a relationship to the amount of the slip of the belt. Namely, it is known that the amount of slip becomes larger as the remaining service life of the driving belt becomes shorter. Therefore, it is also required to detect the slip of the driving belt correctly in order to predict the remaining service life of the belt and to prevent the trouble caused by breakage of the belt.
Heretofore, various devices have been proposed to detect the slip of driving belt of the generator. For example, Japanese Unexamined Patent Publication (Kokai) No. 49-46054 discloses a slip-detecting device for a driving belt of a generator which is provided with a main detector for detecting a rotational speed of an internal combustion engine and a sub detector for detecting a rotational speed of a generator driven by the engine via a driving belt. The device in '054 publication detects the slip of the driving belt based on an accumulated values of the speed signals from the main detector and the sub detector.
In general, the slip of the driving belt can be easily detected by detecting and comparing the rotational speeds of the driver (the engine) and the driven equipment (the generator). Further, an internal combustion engine is usually provided with an engine speed detector (for example, a crank angle detector) in order to utilize an engine speed as a control parameter of the engine. Therefore, if an internal combustion engine is used as the driver, an accurate engine speed is easily detected.
However, usually, a generator is not provided with a speed detector. Especially, in case of a vehicle engine, since it involves additional cost, it is not preferable to provide a speed detector on the generator which is used exclusively for detecting the generator speed. Therefore, a fluctuation of an output voltage of the generator or a waveform thereof, in lieu of the speed detector, is normally used for calculating the generator speed. The sub detector in the device of the '054 publication generates a pulse signal having frequency proportional to the generator rotational speed by processing the fluctuations (the ripples) of the output voltage by a waveform shaping circuit. Namely, in the '054 publication, the generator speed is detected based on the fluctuations (the ripples) of the output voltage of the generator.
However, if the generator speed is detected based on the ripples in the generator output, it is difficult to detect an accurate generator speed in some cases.
In some generator system, such as in the generator system of a vehicle, a battery is always connected to the generator in order to charge the battery during the operation of the engine. Therefore, in this type of the generator system, the output voltage of the generator is usually controlled so that the output voltage becomes lower than an upper limit (for example, 12 to 14 volts) in order to prevent overcharging of the battery by keeping the electric current supplied to the battery at an adequate level. In this case, since the output voltage of the generator is controlled in a relatively small range, accordingly, the amplitude of the ripples becomes small. Further, since the battery is always connected to the generator, the ripples in the output voltage are smoothed by the battery and, thereby, the amplitude of the ripples in the actual output voltage become very small.
Therefore, in the device of the '054 publication, a pulse signal accurately corresponding to the rotational speed of the generator is not obtained by waveform shaping due to the small amplitude of the ripples. Thus the generator speed calculated from the pulse signal is sometimes inaccurate in the device of the '054 publication and, thereby, the slip of the belt is not detected correctly in some cases.